A PRESENTION BY

ABDULLAHI NAZIFI ABDULLAHI

(MS bioengineering student )

AT CYPRUS INTERNATIONAL

UNIVERSITY (CIU)

2014

Introduction

Flotation principal and process description

Flotation Biological System

Operational Difficulties and Remedy

summary

Conclusions

References

Flotation process (sometimes called flotation separation) is a method of separation widely used in the wastewater treatment and mineral processing industries.

Activated sludge consists of biological flocs that are matrices of microorganisms, nonliving organics, and inorganic substances.

The microorganisms include bacteria, fungi, protozoa, rotifers , viruses, and higher forms of animals such as insect larvae, worms, and crustacean.

Dissolved air flotation is mainly used to float suspended and colloidal solids by decreasing their apparent density.

The flotation system consists of eight major components: Influent feed pump, air supply, pressurizing pump , air dissolving tube(retention tank),friction valve, flotation chamber , spiral scoop, and effluent extraction pipe.

Figures1 and 2 show a single cell and a double cell, respectively, of high rate dissolved air flotation clarifiers.

The inlet, outlet, and sludge removal mechanisms are contained in the central rotating section.

In this section the spiral scoop rotate around the tank at a speed synchronized with the flow.

Unclarified water, first passing through an air dissolving tube and a friction valve, is released through a rotary joint in the center of the tank.

It then passes into the distribution duct that moves backward with the same velocity as the forward incoming water.

The settling and the flotation processes take place in the quiescent state in the flotation chamber.

The spiral scoop takes up the floated sludge, pouring it into the stationary center section where it is discharged by gravity for either recycling or disposal.

Clarified water is removed by effluent extraction pipes, which are attached to the moving center section.

Wiper blades attached to the moving distribution duct scrape the bottom and the sides of the tank and discharge settled sludge into the built in sump for periodic purging.

The variable speed gear motor drives the rotating elements and scoop.

Air dissolving tube

Activated sludge is a continuous flow biological treatment process characterized by the suspension of aerobic microorganisms, maintained in a relatively homogeneous state by the mixing and turbulence induced by aeration.

The microorganisms oxidize soluble and colloidal organics to CO2 and H2O in the presence of molecular oxygen.

The process is generally but not always , preceded by a primary sedimentation clarifier.

The mixture of microorganisms and wastewater formed in the aeration basins, called mixed liquor, is transferred to gravity clarifiers for liquid solid separation.

The major portion of the microorganisms settling out in the clarifiers can be recycled to the aeration basins to be mixed with incoming wastewater, while the excess, which constitutes the waste sludge, is sent to the sludge handling facilities .

The rate and concentration of activated sludge returned to the aeration basins determines the mixed liquor suspended solids (MLSS) level developed and maintained in the basins.

During the oxidation process, a certain amount of the organic material is synthesized into new cells

some of which then undergoes auto-oxidation (self-oxidation, or endogenous respiration) in the aeration basins, the remainder forming net growth or excess sludge.

Oxygen is required in the process to support the oxidation and synthesis reactions.

Volatile compounds are driven off to a certain extent in the aeration process.

Metals will also be partially removed and accumulated in the sludge.

Activated sludge systems are classified as high rate, conventional, or extended aeration (low rate) based on the organic loading.

In the conventional activated sludge plant, the wastewater is commonly aerated for a period of 4–8h (based on average daily flow) in a plug flow hydraulic mode.

Either surface or submerged aeration systems can be employed to transfer oxygen from air to wastewater.

The most common operational difficulties encountered in the conventional activated sludge treatment plant are rising sludge and bulking sludge, resulting in high suspended solids and 5-day BOD in the plant effluent.

The common cause of rising sludge is biological denitrification, in which nitrites and nitrates in the wastewater are converted to nitrogen gas.

When enough nitrogen gas is formed and trapped in the sludge mass, the sludge in the conventional secondary sedimentation clarifier becomes buoyant and floats to the surface.

This phenomenon is called biological flotation.

Rising sludge can also be caused by internal solids overloading and hydraulic overloading to secondary sedimentation.

Poor sedimentation clarifier design and operation in terms of flow-through velocity, weir design, etc. are also possible causes.

Sludge bulking is another phenomenon that often occurs in activated sludge plants whereby the sludge occupies excessive volumes and will not settle rapidly.

There are two principal types of sludge bulking problems:

(a) The growth of filamentous organisms

(b) The formation of swelling biological flocsthrough the addition of bound water to the cells to the extent that their density is reduced

Possible causes of sludge bulking include the following:

(a) Absence of certain necessary trace elements in wastewater

(b) Wide fluctuations in wastewater pH

(c) Limited dissolved oxygen in the aeration tank

(d) Inadequate food-to-microorganism ratio (F/M)

(e) Inadequate mean cell residence time (θc)

(f) Inadequate return sludge pumping rate

(g) Internal plant overloading

(h) Poor sedimentation clarifier operation The problems of sludge rising and sludge bulking, when serious, cannot be overcome easily.

If rising and bulking conditions continue to persist after all the aforementioned factors have been checked, a critical investigation of the behavior of aeration basin and secondary sedimentation clarifier should be made.

It is very possible that the design is at fault, and either changes or expansions must be made in the facilities.

In summary, although the present conventional activated sludge process has been in use for many decades, there is still a lot of room for improvement.

One big area of weakness of the conventional process is the secondary sedimentation clarifier, which gives low consistency sludge, shocks the living biota by holding them for long periods in anaerobic conditions, and has problems of sludge rising and sludge bulking.

The use of a high rate dissolved air flotation unit in series before the final sedimentation clarifier eliminates these deficiencies.

The net results are lower solids loading to the existing sedimentation clarifier, higher hydraulic capacity and retention time of aeration tank, easier concentration of waste sludge, more active recycled sludge, better effluent quality and lower wastewater treatment costs.

Besides, there will be no sludge rising or sludge bulking problems

The flotation cell is a high rate DAF clarifier.

The unit has minimum volume (less than 16in. water depth), low cost and flexibility in application due to its small size.

It has been very successfully applied for in-process and secondary flotation in industrial and municipal applications.

The flotation cell has the following advantages when used to separate biological solids from activated sludge mixed liquor:

(a) Sludge consistency for the floated biological sludge is about three times higher than comparable settled sludge.

This fact has been recognized for years in the use of flotation in thickening wastewater sludges.

(b) The floated sludge is an aerobic. There is far less kill of the biological community due to anaerobic shock.

This has been demonstrated in field comparisons using both floated and settled sludges.

(c) The aerobic floated sludge is returned to the aeration basin in 8–15min instead of several hours for the conventional activated sludge process using secondary sedimentation clarifier.

(d) Clarified water from a properly operated and sized flotation unit is comparable in quality to settled effluent.

This has been demonstrated in field pilot trials. Where a settling unit already exists in the treatment process, the practice is to undersize the flotation cell and install it in series with the existing settling unit.

The flotation cell is then the workhorse or “harvester” and the settling unit is the final “polisher” for the effluent.

The problems of high solids loading, high hydraulic loading, sludge rising, and/or sludge bulking in existing conventional activated sludge treatment plant can then be eliminated.

(e) The small size of the flotation cell greatly simplifies the issues involved in the upgrading of existing wastewater treatment plants.

(f) Capital cost of flotation cells is lower than the cost for conventional sedimentation clarifiers or comparable basin expansion.

(g) Stainless steel and prefabricated construction increase the economic feasibility of flotation cells.

THANK YOU ALL

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